Microparticles are commonly used as a seed material in wind tunnel testing to measure the airflow velocity, biological testing, and histological staining, among other applications. Two common measurement techniques in wind tunnel testing include particle image velocimetry (PIV) and laser Doppler velocimetry (LDV). In both techniques, laser irradiation is scattered off incident particles seeded in the flow. In PIV, a camera is used to take pictures of an area of a flow field which is illuminated by a pulsed laser. The position of the seed particles in sequential images is used to determine velocities; enabling planar data analysis of turbulence levels, length scales, coherent flow structures, flow-induced forces and moments. In LDV, two laser beams cross and interfere to create a fringe pattern which is used to determine point-wise velocities from the frequency of light scattered by the particles.
Good seed materials for PIV and LDV must have a small aerodynamic diameter and a high index of refraction. Aerodynamic diameter is an indicator of how well a seed particle velocity matches that of the surrounding fluid. If the aerodynamic diameter is small, the particle velocity can be assumed to be equal to the velocity of the surrounding fluid. Traditional seed materials include smoke, olive oil, kerosene and diethylhexylsebacate (DEHS). Polystyrene latex microsphere particles (PSLs) have also been used extensively as seed materials for PIV and LDV measurements due to their low aerodynamic diameter and high refractive index, which results in higher intensity Mie scattering when they are illuminated with a laser light
For PSLs to be used as seed material in laser-induced fluorescence (LIF) experiments, the integration of highly fluorescent dyes in the PSLs at concentrations great enough to enable detection in the airflow is critical. Several techniques have been utilized to incorporate dye materials in particles and can basically be split into two approaches, incorporation of the dye during particle synthesis and incorporation after particle synthesis. Incorporation of dye into existing particles can be achieved on just the surface through adsorption or into the particle matrix by particle swelling followed by solvent exchange. However, these methodologies do not yield particles with homogeneous distribution, macroscopically, of dye within the particle matrix and require multi-step processes leveraging expensive filtration devices to generate the dye incorporated particles. Additionally, the dyes used in current approaches, such as members of the rhodamine family (e.g., rhodamine 610), have been classified as potentially toxic and carcinogenic by the International Agency for Research on Cancer. Therefore, these dyes are not likely to be used for wind tunnel testing.